Reboot your machine. If Quickbooks is installed on a server or another machine, you MUST reboot your server as well.

With Quickbooks open, press F2 (or Function + F2) and look at the file/path of your Quickbooks company file. It must match the file/path used in the LMN QB Sync tool exactly... If you're using mapped drives to connect with LMN Sync, you should use the real computer names/folders instead

There is a known bug in some versions of Quickbooks. Disable QB Quick Start.

From the Edit menu (in Quickbooks), select Preferences

Choose the General menu and click the My Preferences tab

Clear the Keep Quickbooks Running for quick startups checkbox (should be left unchecked)

Click OK

If these steps fail, you should proceed to making sure all the latest updates for Quickbooks are installed. There are KNOWN ISSUES with some versions of Quickbooks that they fixed in application updates. Make sure you are running the latest and greatest version of your Quickbooks before trying any further.

Here a link to Intuit's site describing how to update to the latest. Please contact Intuit support if you're having any difficult updating your Quickbooks. LMN support staff cannot assist with this process.